Conventionally, a circuit substrate accommodated in a case is fixed to a vehicle for protecting an attachment element on the circuit substrate. JP-A-2011-166048 and JP-A-2005-317692 describe a structure without using a fastener such as screw, so as to reduce a cost when the circuit substrate is fixed to the case.
The above structure basically includes a top case and a bottom case. The circuit substrate is inserted between the top case and the bottom case. A force for holding the circuit substrate is affected by an engagement state between the top case and the bottom case, and thereby the circuit substrate may be loosely received in the top case and the bottom case. Since the circuit substrate cannot be strongly received, one of the top case and the bottom case may be readily apart from the other one by an external force. Thus, a displacement may be caused, and the circuit substrate and the element may be affected by an oscillation applied to the cases for a long time.
Further, an acceleration sensor or a gyro sensor may be attached to the circuit substrate. For example, an acceleration sensor for two directions is attached to an airbag electronic control unit (airbag ECU). Since the sensor on the circuit substrate is for measuring parameters relative to movement applied to the vehicle, a no-displacement state is necessary to be maintained between the substrate and the cases. When the displacement occurs, the sensor cannot correctly measure the parameters, and an abnormality such as an erroneous detection may be caused. For example, a security device may activate when the security device should not be activated.